The export duty is only part of the problem. Another part is oil extraction tax. This tax is based on the international price of oil but must be paid even on the amount sold internally at much lower price. So, the more the international price of oil, the less profitable is internal sales. Many old wells are abandoned due to this problem.

I believe Putin did say over the last few years that taxes and tariffs would be adjusted to ensure that Russia always had enough oil "for home use" versus export. The Kremlin is attempting to manage its resource base, though we might not like how they are doing it.